The concept of a movable gate Field Effect Transistor (FET) has been extensively studied and reported in literature. A number of devices have been disclosed that make use of a movable gate FET for building accelerometers.
Force balance feedback control of vibration sensors has been used in seismometers and in accelerometers for attaining increased bandwidth and dynamic range. A number of devices using this approach have been routinely demonstrated and its theory of operation is well understood. Several seismometer and accelerometer manufacturers base their designs on this principle. The fabrication of a silicon accelerometer using wafer bonding techniques is disclosed in great detail in U.S. Pat. No. 5,095,752 and U.S. Pat. No. 5,417,312. In these invention disclosures a relatively large mass made of silicon is encapsulated in a cavity formed by electrodes made out of glass on silicon. The accelerometer is operated using an active feedback loop, in which control voltages are applied to the upper and lower electrodes. The displacement of the free mass under acceleration requires compensation through changes in the voltage applied between the upper and lower electrodes and the moving mass.
Utilizing the FET concept, the feedback bulk silicon micromachined accelerometer disclosed in U.S. Pat. No. 5,205,171 makes use of a feedback loop and at least one pair of dual electrodes acting as capacitive transducers. Differential sensing of the beam-gate capacitance variations is used to generate a null feedback signal used to modulate the voltage applied to electrodes in order to prevent them from moving. Based on the similar concept of using a FET, a device with acceleration dependent gain is disclosed in U.S. Pat. No. 5,103,279 and a device that uses a piezoelectric device to generate voltage for the gate is disclosed in U.S. Pat. No. 4,873,871.
The type of accelerometers mentioned above have good sensitivity at low frequencies but limited sensitivity at high frequencies, although they result in higher bandwidth devices than open loop devices.